The invention relates to a method and a transmitting device for transmitting data symbols from subscriber signals via a radio interface of a mobile communications system, for example for base stations or mobile stations in mobile radio networks.
Various transmission methods are disclosed in various references. These references include an article by E. Sourour et al. entitled xe2x80x9cPerformance of Orthogonal Multi-Carrier CDMA in a Multipath Fading Channelxe2x80x9d, Proceedings of the Global Telecommunications Conference (Globecom), IEEE, San Francisco, Nov. 28, 1994, Volume 1, pp.390-392; FR 2 737 366 A; an article by Rolando entitled, xe2x80x9cRadio Characteristics of the CTDMA and OFDM Techniques and Their Possible Applications to Future Mobile Services (UMTS)xe2x80x9d, CSELT Technical Reports, Volume XXIV, No. 1, February 1996, pp.149-164; International Patent Application WO 96 17 455 A and Published, European Patent Application EP 0 752 765 A.
Code division multiple access (CDMA) radio systems with a plurality of carriers are known from a reference by G. Fettweis, A. S. Bahai, and K. Anvari, titled xe2x80x9cOn Multi-Carrier Code Division Multiple Access (MC-CDMA) Modem Designxe2x80x9d, Proceedings of the IEEE 44th Vehicular Technology Conference VTC""94, Stockholm, 1994, pages 1670-1674. When such systems are used for mobile communication, there is a radio interface between fixed-position base stations and moving mobile stations. The transmission path from a base station to a mobile station is called the downlink, and the transmission path from a mobile station to a base station is called the uplink.
However, the CDMA radio system with a plurality of carriers according to the prior art is difficult to implement in mobile communications systems since the production of the transmitted signals at the transmission end, which transmitted signals contain data symbols allocated to subscriber signals, cannot be adequately matched to the transmission conditions of the radio interface.
It is accordingly an object of the invention to provide a method and a transmitting device for transmitting data symbols from subscriber signals via a radio interface of a mobile communications system which overcome the above-mentioned disadvantages of the prior art methods and devices of this general type.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for transmitting data originating from subscriber signals via a radio interface of a mobile communications system, and N data symbols of a subscriber signal form a block, the method which includes:
dividing the block into a plurality of partial blocks each having Ns data symbols;
allocating the Ns data symbols to sub-carriers;
modulating in parallel the Ns data symbols onto in each case at least one of the sub-carriers for simultaneous transmission, a modulation for each of the sub-carriers being carried out using at least one individual code symbol;
heterodyning the sub-carriers to form a broadband carrier; and
transmitting the plurality of partial blocks via the radio interface.
In the case of the method according to the invention for transmitting data symbols that originate from one or more subscriber signals, the radio interface of the mobile communications system is presupposed and uses subscriber separation, for example in accordance with the CDMA method. In this case, N data symbols of a subscriber signal form a block. Such a block is, for example, a radio block as is transmitted within one timeslot in time division multiple access (TDMA) systems.
In a first method step, the block is divided into a plurality of partial blocks with Ns data symbols. The Ns data symbols are then allocated to sub-carriers and are modulated in parallel onto these various sub-carriers, the modulation for each of the sub-carriers is carried out with at least one individual code symbol. The sub-carriers are heterodyned to form a broadband carrier, so that the Ns data symbols are transmitted simultaneously. The transmission is then carried out in N/Ns successive partial blocks via the radio interface.
The described transmission method has a corresponding receiving method performed at the receiver end. This results in the radio interface having a system structure that is highly resistant to interference as a result of the advantages of the CDMA subscriber separation (with modulation/spreading of data symbols using different code symbols), for example frequency diversity and interference diversity, and which, by using the multi-carrier method with the aid of a plurality of sub-carriers, allows flexible allocation of frequency resources. The method according to the invention allows multiple subscriber interference and inter-symbol interference to be taken into account and compensated for in an appropriate manner.
The method according to the invention makes it easier to achieve cost-effective implementation of flexible CDMA-mobile communications systems. The signal production for the subscriber separation with DS (direct sequence) CDMA and by multiple-carrier methods is combined according to the invention. The transmission of the data symbols can use the specific advantages of both methods flexibly and with a large number of degrees of freedom.
According to an advantageous development of the invention, the sub-carriers are heterodyned linearly to form a broadband carrier. The complexity of signal production and of detection after transmission is thus kept low.
According to a refinement of the invention, one data symbol is transmitted on a plurality of sub-carriers. This ensures frequency diversity for the data symbol, making the transmission more interference-resistant. In addition, it is advantageous to disposed unused frequency bands between the sub-carriers for the data symbol or groups of sub-carriers. Such interleaving of the frequency bands for different data symbols or subscriber signals results in transmission errors being scattered, and the capability to compensate for them more easily at the receiver end.
If, for a given range of allocated frequencies, sub-carriers are allocated to a partial block or to a data symbol in such a way as to maximize the interval between the center frequencies of the sub-carriers, this results in the best possible frequency diversity.
The flexibility of the method according to the invention is advantageously used, in particular, if the number of data symbols in a partial block can be varied depending on the transmission conditions of the radio interface. In this way, the bit rate can also be adapted, depending on the requirements or the maximum permissible values in the transmission conditions.
It is likewise advantageous to be able to vary the number Q of sub-carriers allocated to one data symbol, depending on the transmission conditions of the radio interface. This makes it possible to match the interference immunity to the transmission conditions and at the same time to manage the frequency resources economically.
According to a further refinement of the invention, a period which is intended for transmission of the data symbol can be varied depending on the transmission conditions of the radio interface by varying the number of simultaneously transmitted data symbols in the partial block. This allows the advantages of the multi-carrier or of the code multiplex method to be used depending on the length of the period and/or the number of simultaneously transmitted data symbols. This can be done for a constant data rate; the radio interface parameters are set individually, depending on the specific transmission conditions.
If a guard time without any transmission information is provided within a period that is intended for transmission of the data symbol, then particularly simple and economic receivers can be used at the receiver end. This option is feasible when the periods set for the data symbol are relatively long.
According to the invention, it is possible to select the data symbols without any limitations from the set of complex numbers since, owing to the sub-carrier-related modulation, a large number of individual modulation types and a large and easily variable range of combinations of data symbols and code symbols can be selected.
In addition to CDMA subscriber separation, the method according to the invention can also provide subscriber separation in accordance with a TDMA and/or FDMA method, so that it can easily be implemented in existing mobile radio networks.
The modulation is prepared in a sub-carrier-related manner by using a memory device for storing values of individual codes in matrix form, these values including any required linear transformation of the data symbols to form modulated data symbols and/or pulse shaping of the data to be transmitted, by weighting.
The control device advantageously allow symbol-related individual processing of the data symbols to be carried out by allocating a different number of sub-carriers to at least two data symbols in a block, or by dividing a block into at least two partial blocks with a different number of data symbols. The data symbols in a block may have different importance. Thus, for example, interference with signaling symbols is normally more serious than with symbols for voice transmission. The method according to the invention can take account of this in an excellent manner, since the symbols or the partial blocks are dealt with individually.
The interference immunity can be described by the product of the bandwidth of the sub-carriers and the transmission duration for a symbol so that appropriate allocation of resources (number of sub-carriers and symbol duration) of the radio interface within a block defines an individual interference immunity.
A control device in the transmitting device is advantageously configured in such a manner that the symbol-related individual processing of the data symbols is switched over in accordance with the stipulations of a device for radio resource management, depending on the transmission conditions and/or the load level of a radio cell. This ensures the flexibility to satisfy the requirements, even during operation.
According to the invention, it is possible to use existing frequency bands of a GSM mobile radio network or of another mobile radio network, for example PDC, IS-54/136, PHS, DECT or IS-95, for one or more frequency bands of a sub-carrier, and/or to dispose the frequency bands between sub-carriers.
The transmitting device may be used in the base stations or the mobile stations. A major advantage of the transmission method according to the invention is that conventional detection methods can be used for the corresponding receiving devices, and it is thus possible to match the specific capabilities of the communication partner by simple parameter setting in the transmitting device.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and a transmitting device for transmitting data symbols from subscriber signals via a radio interface of a mobile communications system, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.